Alcohol and drug use increase the risk for violence, HIV, and injuries. Excessive alcohol use is a leading cause of preventable death, associated with over $200 billion in economic costs. Thus, it is a priority to understand the determinants of alcohol and drug use, particularly in youth. Low overall serotonergic (5-HT) functioning plays a role in the etiology of alcohol and drug involvement. However, the mechanisms underlying this relation are less clear as very few longitudinal studies on this topic have been conducted. Understanding the early mediating mechanisms in this relation would be informative for the etiology, prevention and intervention of alcohol and drug involvement. Measured genetic data can be leveraged to model variability in overall 5-HT function. Unfortunately, candidate gene studies have widely recognized limitations, including lack of replication, small effect sizes, and poor understanding of biological mechanisms. To address these gaps and limitations, the proposed study will prospectively examine childhood temperament characteristics and early adolescent psychopathology as potential mechanisms of influence in the relation between low overall serotonergic functioning and mid/late adolescent alcohol and drug involvement. It will test these models in two longitudinal studies of adolescents and will combine the results and account for heterogeneity across studies using meta-analytic techniques. Aim 1 will model genetic risk for low 5-HT function by using information from multiple single nucleotide polymorphisms (SNPs) to create a biologically based genetic risk score. SNPs in 5- HT genes will be selected for inclusion in the risk score by using bioinformatics tools, like the Encyclopeda of DNA Elements, to identify SNPs with functional effects on gene expression and/or regulation. Aim 2 will provide the first prospective test of childhood effortful control, sadness, and anger as moderated mediators in the relation between a genetic risk score indexing low 5-HT function and mid/late adolescent alcohol and drug involvement. Aim 3 will provide the first prospective test of early adolescent pure conduct problems (CP), pure depressive symptoms (DEP), and co-occurring CP/DEP as mediators in the relation between a genetic risk score indexing low 5-HT function and mid/late adolescent alcohol and drug involvement. Aim 3 will account for methodological limitations of previous studies by simultaneously accounting for art factual CP/DEP comorbidity and controlling for baseline alcohol/drug involvement and other adolescent psychopathology. The study has implications for understanding developmental pathways to alcohol and drug involvement and will be the first to prospectively study childhood temperament and early adolescent psychopathology as mechanisms of risk in the relation between a genetic risk score indexing low 5-HT function and alcohol and drug involvement in youth.